Within telecoms communication systems, light entering an input port of a device may have any possible polarisation state and this may be time varying. If such light is routed through a device, it is preferable that the light that appears in the output port has an amplitude, which does not depend on the input polarisation state.
In order to allow operation with modulated signals, a device or system preferably ‘honours’ polarisation, i.e., paths of beams of different polarisation are matched in order to allow operation without disturbance of relative polarisation between the beams of different polarisation. For example, polarisation insensitive phase modulation may ensure that the intensity of steered or routed output beams are substantially unaffected by the polarisation state of the input beam. More particularly, polarisation insensitive phase modulation may provide a known, minimal and/or time independent insertion loss.
An optical device or system may use Liquid Crystals (LC). The liquid crystalline molecules in a material exhibiting a nematic or ferroelectric LC phase are typically rod shaped. The direction of preferred orientation of such LC molecules in the neighbourhood of any point can be represented by n (a dimensionless unit vector), where n and −n are fully equivalent. In other words, an LC device has a liquid crystal director that can be regarded as an arrow, which indicates the average preferred orientation of liquid crystalline molecules in a liquid crystal material. Both directions (180 degrees apart) of the arrow are equivalent.
Ferroelectric liquid crystalline (Sc*) materials switch largely in the plane of a Liquid-Crystal-on-Silicon (LCOS) device. When an electric field is applied to such a device, the director changes its orientation in the plane of the device. An array of pixels switched such that one of two opposite positions are taken around the director cone produces (under certain conditions) an array with binary phase levels 0 and pi. Such a binary phase array produces diffraction peaks in the device output plane that include both the first order (routing) peak and unwanted higher and symmetric orders which results in power loss and potential crosstalk. The loss of a device using this effect will not depend on the input polarisation state, i.e. it is polarisation insensitive, but will depend on the LC layer thickness and the switching angle.
LC may be provided in a Liquid-Crystal-on-Silicon (LCOS) device having the liquid crystalline material on a silicon substrate that is coated with a reflective layer. LC applied to such a reflective substrate may be controllable to allow light to be reflected or blocked. Specifically, a LCOS may comprise a silicon CMOS chip having a reflective coating (e.g., comprising aluminium) that is covered with LC, and a glass layer over the LC.
The field of optical beam steering continues to provide a need for improvements such as, for example, reduced insertion loss and/or polarisation insensitivity.
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